The proper functioning of various systems and apparatus relies upon an ability to position an object, such as a workpiece, accurately and precisely, such as relative to a machining tool, processing tool, or imaging device. Object placement is perhaps most critical in lithographic exposure systems used in the fabrication of microelectronic devices, displays, and the like. These systems, called microlithography systems, must satisfy extremely demanding criteria of image-placement, image-resolution, and image-registration on the lithographic substrate. For example, to achieve currently demanded feature sizes, in projected images, of 100 nm or less on the substrate, placement of the substrate for exposure must be accurate at least to within a few nanometers or less. Such criteria place enormous technical demands on stages and analogous devices used for holding and moving the substrate and for, in some systems, holding and moving a pattern-defining body such as a reticle or mask.
The current need for stages capable of providing extremely accurate placement and movement of reticles, substrates, and the like has been met in part by using laser interferometers for determining stage position. Microlithography systems typically use at least two perpendicular sets of laser interferometer beams to measure the horizontal (x-y) two-dimensional position of an x-y stage. The stage and interferometer system are enclosed in an environmental chamber containing a flow of highly filtered and temperature-controlled air, in part to prevent deposition of particulate matter on the lithographic substrate or on the reticle. The environmental chamber thus assists in maintaining the index of refraction of the air at a substantially constant value by maintaining constancy of the air temperature.
In many types of microlithography systems, a projection-optical system (“projection lens”) is situated between a reticle stage and a substrate (wafer) stage. The projection lens is rigidly mounted on a rigid, vibration-isolation support to suppress motion of the projection lens. The projection lens must remain very still during the making of lithographic exposures from the reticle to the substrate. However, the projection lens may exhibit a small amount (typically several nanometers or less) of motion caused mainly by vibrations. Among various sources of these vibrations are circulation of coolant in the projection lens (which is temperature-regulated in this manner), reactionary forces to stage motion, and the like. These movements cause corresponding changes in the length of the propagation pathway of the reference beam. Consequently, data obtained by the measurement beam are uncorrected with respect to lens motion. As the performance standards of microlithography systems become stricter, reducing the effects of these motions on position measurements is becoming more important.
In view of the importance of aligning the stages very accurately with the projection lens, the projection lens is used as a reference body for determining the position of the stage. In other words, the respective position of each stage is determined relative to the projection lens. For such a purpose, reference mirrors for reflecting reference interferometer beams are mounted to the column containing the projection lens. Usually, two reference mirrors (at right angles to each other) are provided on the projection lens, one for reflecting x-direction reference interferometer beams and the other for reflecting y-direction reference interferometer beams.
This scheme is illustrated in FIGS. 6(A)-6(C), showing a projection lens 202, a stage 204 (e.g., wafer stage), one x-direction “fixed” reference beam 206 produced by an x-direction reference interferometer 208, and two y-direction reference beams 210, 212. The x-direction reference beam 206 is incident on the mirror 214, and the y-direction reference beams 210, 212 are incident on the mirror 216. The mirrors 214, 216 are at right angles to each other and are mounted on or at least associated with the projection lens 202. Associated with the x-direction reference beam 206 is an x-direction measurement beam 218, produced by an x-direction measurement interferometer 220, incident on a mirror 222 on the stage 204. Similarly, associated with each y-direction reference beam 210, 212 is a respective y-direction measurement beam (not shown) incident on the stage 204. These two y-direction measurement beams are used for detecting yaw of the stage 204 (i.e., motions of the stage about the axis Ax extending in the z-direction).
Additional interferometer beams may be present to provide corrections to the stage position from other motions of the stage, such as pitch, roll, or height. These will not be considered here explicitly.
Stage position in the x-direction, for example, can then be corrected for small motions of the lens, by subtracting the lens x-position, determined from the x-direction reference beam 206, from the stage x-position. If the stage is traveling purely in the x-direction, the length of the x-direction reference beam 206 can be subtracted directly from the x-direction measurement beam 218. If the stage motion is not purely in the x-direction, the length of the x-direction reference beam 206 is subtracted from the x-displacement component, which is calculated from measurement information obtained from the stage-measurement interferometers. This correction method assumes any changes in the path-length of the x-direction reference beam 206 are caused by motion of the projection lens. However, if the optical path-length of the x-direction reference beam 206 changes because the optical properties of the ambient atmosphere change, an erroneous correction to the position of the projection lens will be produced.
Furthermore, any fluctuations in the optical path-length of the x-direction measurement beam 218, from changes in the optical properties of the ambient atmosphere, will cause further errors in the stage position.
The adverse effects of air currents and air-density fluctuations on interferometer beams are known. For example, air experiencing local variations in temperature exhibits corresponding variations in density and refractive index. If air turbulence is occurring in the propagation pathway of an interferometer beam, the turbulence can mix regions, or cells, of air of different refractive indices, producing changes in the optical path length of the beam, which degrade the accuracy and precision of positional measurements determined by the interferometer. Various approaches have been adopted to address this problem, notably by enclosing the stages and interferometers in an environmental chamber, as noted above, and by producing and maintaining improved (gentle laminar flow and constant temperature) air circulation in the vicinity of the interferometers and stages. Exemplary approaches are discussed in, for example, U.S. Pat. No. 4,814,625 to Yabu, U.S. Pat. No. 5,141,318 to Miyazaki, and U.S. Pat. No. 5,870,197 to Sogard et al., all incorporated herein by reference. In general, referring again to FIGS. 6(A)-6(C), the corresponding reference and measurement beams 206, 218 are situated as close as possible to each other and have similar respective lengths. The beams 206, 218 are situated in a stream of air (arrows 224) flowing from the reference beam(s) to the measurement beam(s). The air flow 224 is usually at right angles to the beams 206, 218. However, these approaches do not completely eliminate the problem of air-density fluctuations in the beam paths of the interferometers.
Therefore, there is a need for devices and methods for, in the context of interferometrically measuring position of a stage, correcting for fluctuations in the optical path lengths of the interferometer beams.